Rolling it just so

Walk onto the floor at BEPeterson (BEP) and nothing looks particularly out of the ordinary for a contract manufacturer specializing in large fabrications. There' the 53-foot-long, 13-ft.-wide plasma table; large vertical turning centers; and arc welding machines. But a closer look at one rolling system reveals that this isn't a typical operation.

Adjacent to the plasma table is a four-roll machine that processes between 100 and 200 parts a week. The parts require unusually tight tolerances: ±1/32 in. on the circumference for cans up to 70 in. in diameter (see Figure 1). It processes up to 3/16-in.-thick stainless steel and up to 3/8-in.-thick aluminum—not just any aluminum, but some of the softest made for these kinds of applications: 1100-0.

A first glance at these specs indicates that these requirements call for two machines, one with crowning for the stainless steel and one without for the soft aluminum. Managers were ready, in fact, to buy two machines, but they ultimately found they were able to purchase just one that was highly customized and, most important, oversized for the job.

Precision Shop, Precision Work

The job involves rolled shells that make up complicated, tight-tolerance medical equipment subassemblies. BEP, an 80-employee contract manufacturer in Avon, Mass., south of Boston, produces several hundred of these components a month in an area dedicated to contract jobs in the medical equipment and commercial water filtration fields. These two sectors call for products that require some of the same fabricating operations: high-definition and conventional plasma cutting, bending with some heavy-duty press brakes, welding, and, of course, rolling.

But as Sales Account Manager John Adams explained, there' another reason these product lines share a space. "The industries seem to share the same mentality," he said. "It's clean work, with lots of finesse." Every product has to be just so, including medical equipment components made of hard stainless steel and soft aluminum.

For all but the softest-metal applications, rolls are usually crowned, with a slightly thicker diameter in the middle than on the sides, to handle the deflection as the metal rolls through. But crowning for soft aluminum isn't necessary and can even stretch the center of the sheet more than the sides, meaning the edges wouldn't meet up quite right at the end of the roll."This wasn't acceptable for these components," Adams said, "especially considering the tolerances we work with."

Before the company brought in its current CNC rolling system, operators used an 8-ft., manual, four-roll machine, but had to swap the top crowned roll for a straight one to handle the soft aluminum. The method sufficed for several years, but it certainly hindered efficiency. So the company picked up an unusual machine, one oversized for the material and with no crowning. The machine is a Faccin model 2531 (25 is 2.5 meters, or about 8 ft. wide; 31 stands for 31 cm, or about 12-in.-diameter rolls).

No More Crowning

The job calls for several requirements that make this specific plate rolling application especially unusual. One, the rolls are large enough so they can handle the stainless without crowning, which would distort the soft aluminum.

So why, exactly, does oversizing the roll eliminate the need for crowned rolls? Steve Bonnay, the Tampa, Fla.-based product manager for Faccin, an Italian company, used a dock analogy to explain. An average man walking out to a dock platform consisting of 2-by-4s may experience some bend, or "give," in the wood. "Instead of 2-by-4s, try building that dock platform with a 12- by 12-foot block of oak. The man would walk out there and nothing would happen. It would be rock-solid."

In most rolling applications, crowning allows fabricators to get away with 2-by-4s instead of that expensive block of oak. As, say, 1/4-in. steel plate rolls through the machine, it creates deflection that causes the rolls to push out slightly in the center. Like on a press brake, deflection is most pronounced in the center, the farthest away from the rigid ends attached to the machine base. A slight bulge, or crown, in the center of the roll accounts for that slight deflection, so the position of bend from one side to the other remains true. In reality, the same logic behind press brake crowning applies to crowning in roll bending.

Extreme Diameter Control

The second unusual set of features relate to the extreme accuracies. Like many newer systems, the machine has a CNC, but this one has a digital readout to three decimal places. "Because our applications call for a tolerance of 1/32 inch, we needed to make incremental changes smaller than that," Adams said, "so the operator can fine-tune it to get it just right."

Getting it just right requires some training, because even though these applications use aluminum and stainless, material variation remains an issue because the tolerances are so tight. Every month the company purchases coil in mill-run quantities, and thickness and hardness can vary from coil to coil, not to the extent it does for, say, A36 steel, but enough to make springback variation an issue and throw the rolling operation off by more than 1/32 in. So roll values (the pressure exerted by the rolls) must change ever so slightly—which is why that third decimal place in the digital readout matters.

Extreme accuracy also calls for another unusual rolling requirement: All four rolls are driven. It' not unusual to drive the top (pinch) and bottom center rolls. And many pressure vessel shops, wind tower manufacturers, and similar firms use systems with three drive rolls: the top, bottom, and outboard roller (the one opposite the material feed side). The third drive roll provides a little extra "grab" for shops that may store plate out in the elements and don't shotblast or otherwise treat it. Oily or rusty plates have less chance of slipping in these three-drive-roll systems.

The third drive roll also acts as a safeguard in certain applications, Bonnay explained. The machine allows for prebending. Rolls must grab the lead-in plate edge, an initial pinch point that' not bent but remains flat. Prebending eliminates the narrow, flat portion by feeding the plate in, then out and back in again. But this doesn't prevent the system from leaving an ever-so-narrow flat portion at the end of the plate, because the machine has to hold on to the metal somewhere at the end of the roll bend.

This is where re-rolling comes into play—shops roll, weld, grind the weld down, and then roll it again to get that last bit of flatness out of the can. If someone forgets to grind down the weld line and then places it back into a two-drive-roll machine, catastrophe can result as, say, a 1/2-in.-thick weld line is jammed through a 1/4-in. roll opening. A third drive roll can prevent a system crash because "you're not loading your material as much on a single point," Bonnay said.

A fourth drive roll is necessary only when extreme accuracy is required, and BEP's +/-1/32-in. specification certainly qualifies. Besides greater control and flexibility, the four drive rolls also allow the rolling pressure to be spread out to the maximum extent. As Bonnay explained, about 60 percent of the pressure comes from the top and bottom rolls, while the remaining 40 percent is divided evenly between the outboard and feed-side rolls.

He reiterated that such an arrangement makes sense only if a shop either works with extremely soft metal, such as aluminum or (even more of a rarity) brass, extremely tight-diameter parts (which are more likely to slip during the operation), or has extraordinarily tight tolerances. BEP's rolling operation has two of those three—tight tolerances and soft metal.

The rolling machine' guidance system—the tables that support the can on either side and the roll support on top—is about as rare as having four drive rolls. Even rarer is how these supports, which articulate, contact the can being rolled in up to five places. As the material is rolled through, entering from the right, the left support table rises and contacts the metal at about the 7:30 position. As the roll continues up, the table articulates in the middle to contact the work again at 10 o'clock. The top support, held on a column connected to the machine base, then grabs the material at noon, before the opposite guidance table contacts the roll at 2 o'clock and then at 4:30, guiding it ever so carefully to the 360-degree point (see Figure 1 and Figure 2).

All this prevents the material, particularly the soft aluminum, from distorting because of the effects of gravity. Without those supports, the can diameter would grow on the side and shrink on top—not by much, but enough to bring this application out of tolerance.

"In most cases," Bonnay said, "trying to hold 1/32-in. [tolerance] is a pipe dream."

This didn't stop BEP from giving it a try—and succeeding.